The present invention relates to a variable-valve-actuation (VVA) apparatus for an internal combustion engine, which can change, particularly, the valve lift of an intake or exhaust valve in accordance with the engine operating conditions.
One of the VVA apparatus is shown in JP-A 11-107725. This VVA apparatus, applied to intake valves, comprises a crank cam arranged on the outer periphery of a driving shaft rotated together with a crankshaft and having an axis eccentric to an axis of the driving shaft, and a valve operating (VO) cam to which torque of the crank cam is transmitted through a transmission mechanism to have a cam face coming in slide contact with the top face of a valve lifter arranged at the upper end of the intake valve for operation thereof.
The transmission mechanism includes a rocker arm disposed above the VO cam and swingably supported to a control shaft, a crank arm having an annular base engaged with the outer peripheral surface of the crank cam and an extension rotatably connected to a first arm of the rocker arm through a pin, and a link rod having a first end rotatably connected to a second arm of the rocker arm through a pin and a second end rotatably connected to an end of the VO cam through a pin.
Moreover, fixed on the outer peripheral surface of the control shaft is a control cam having an axis eccentric to an axis of the control shaft by a predetermined amount and rotatably fitted in a support hole formed substantially in the center of the rocker arm. The control cam changes a rocking fulcrum of the rocker arm in accordance with the rotated position to change the position of contact of the cam face of the valve operating cam with respect to the top face of the valve lifter, carrying out variable control of the valve lift of the intake valve.
Specifically, when the engine operating conditions are in the high-rotation and high-load range, in order to urge an actuator to rotate the control cam in one direction through the control shaft for rotation of the control cam in the same direction, the rocking fulcrum of the rocker arm is moved to approach the driving shaft. Then, an end or a cam nose of the VO cam is pushed downward by the link rod, etc. to move the position of contact of the cam face of the VO cam with respect to the top face of the valve lifter to a lift section of the cam face. Thus, the intake valve is controlled to have the maximum valve-lift characteristic.
On the other hand, when the engine operating conditions are in the low-rotation and low-load range, the actuator rotates the control shaft in another direction for rotation of the control cam in the same direction, moving the rocking fulcrum of the rocker arm to separate from the driving shaft. Then, the pivotal point of the rocker arm and the link rod is moved upward to draw up the cam nose of the VO cam, moving the position of contact of the cam face of the VO cam with respect to the top face of the valve lifter to separate from the lift section of the cam face. Thus, the intake valve is controlled to have the minimum valve-lift characteristic.
Therefore, the VVA apparatus allows full achievement of the engine performance in accordance with the engine operating conditions, i.e., an improvement in fuel efficiency and in engine output.
With the above VVA apparatus, however, though the valve-lift characteristic can be changed by changing the rocking fulcrum of the rocker arm in accordance with the rotated position of the control cam, a full consideration is not made with regard to the direction of rotation of the control cam, particularly, the direction of rotation from the maximum valve-lift control position to the minimum valve-lift control position, and the position of rotation for minimum valve-lift control. This may raise a problem that a full reduction is impossible in the minimum valve lift due to the direction of rotation of the control cam. Moreover, this may raise another problem that during minimum valve-lift control, the line connecting the axes of first and second end pins of the link rod and the line connecting the axis of the second end pin and the axis of the driving shaft form a straight line to produce locking of the link rod, disturbing smooth rotation of the link rod and the VO cam upon transition of operation of the intake valve from closing to opening.
It is, therefore, an object of the present invention to provide a VVA apparatus for an internal combustion engine, which contributes to an improvement in the engine performance and a smooth operation of the apparatus components.